The Science of Climate Change Explained: Facts, Evidence ...

How do we know global warming is not because of the sun or volcanoes? ... An earlier version of this article misidentified the authors of ... Climate|TheScienceofClimateChangeExplained:Facts,EvidenceandProofhttps://www.nytimes.com/article/climate-change-global-warming-faq.htmlCredit...PhotoIllustrationbyAndreaD'AquinoSupportedbyContinuereadingthemainstoryReadinappByJuliaRosenMs.RosenisajournalistwithaPh.D.ingeology.HerresearchinvolvedstudyingicecoresfromGreenlandandAntarcticatounderstandpastclimatechanges.PublishedApril19,2021UpdatedNov.6,2021LeerenespañolThescienceofclimatechangeismoresolidandwidelyagreeduponthanyoumightthink.Butthescopeofthetopic,aswellasrampantdisinformation,canmakeithardtoseparatefactfromfiction.Here,we’vedoneourbesttopresentyouwithnotonlythemostaccuratescientificinformation,butalsoanexplanationofhowweknowit.Howdoweknowclimatechangeisreallyhappening?Howmuchagreementisthereamongscientistsaboutclimatechange?Dowereallyonlyhave150yearsofclimatedata?Howisthatenoughtotellusaboutcenturiesofchange?Howdoweknowclimatechangeiscausedbyhumans?Sincegreenhousegasesoccurnaturally,howdoweknowthey’recausingEarth’stemperaturetorise?Whyshouldwebeworriedthattheplanethaswarmed2°Fsincethe1800s?Isclimatechangeapartoftheplanet’snaturalwarmingandcoolingcycles?Howdoweknowglobalwarmingisnotbecauseofthesunorvolcanoes?Howcanwintersandcertainplacesbegettingcolderiftheplanetiswarming?Wildfiresandbadweatherhavealwayshappened.Howdoweknowthere’saconnectiontoclimatechange?Howbadaretheeffectsofclimatechangegoingtobe?Whatwillitcosttodosomethingaboutclimatechange,versusdoingnothing?ImageHowdoweknowclimatechangeisreallyhappening?Climatechangeisoftencastasapredictionmadebycomplicatedcomputermodels.Butthescientificbasisforclimatechangeismuchbroader,andmodelsareactuallyonlyonepartofit(and,forwhatit’sworth,they’resurprisinglyaccurate).Formorethanacentury,scientistshaveunderstoodthebasicphysicsbehindwhygreenhousegaseslikecarbondioxidecausewarming.ThesegasesmakeupjustasmallfractionoftheatmospherebutexertoutsizedcontrolonEarth’sclimatebytrappingsomeoftheplanet’sheatbeforeitescapesintospace.Thisgreenhouseeffectisimportant:It’swhyaplanetsofarfromthesunhasliquidwaterandlife!However,duringtheIndustrialRevolution,peoplestartedburningcoalandotherfossilfuelstopowerfactories,smeltersandsteamengines,whichaddedmoregreenhousegasestotheatmosphere.Eversince,humanactivitieshavebeenheatingtheplanet.Weknowthisistruethankstoanoverwhelmingbodyofevidencethatbeginswithtemperaturemeasurementstakenatweatherstationsandonshipsstartinginthemid-1800s.Later,scientistsbegantrackingsurfacetemperatureswithsatellitesandlookingforcluesaboutclimatechangeingeologicrecords.Together,thesedataalltellthesamestory:Earthisgettinghotter.Averageglobaltemperatureshaveincreasedby2.2degreesFahrenheit,or1.2degreesCelsius,since1880,withthegreatestchangeshappeninginthelate20thcentury.LandareashavewarmedmorethantheseasurfaceandtheArctichaswarmedthemost—bymorethan4degreesFahrenheitjustsincethe1960s.Temperatureextremeshavealsoshifted.IntheUnitedStates,dailyrecordhighsnowoutnumberrecordlowstwo-to-one. Whereitwascoolerorwarmerin2020comparedwiththemiddleofthe20thcentury –1˚C 0˚ +1˚ +2˚ +3˚ Nodata Whereitwascoolerorwarmerin2020comparedwiththemiddleofthe20thcentury –1˚C 0˚ +1˚ +2˚ +3˚ Nodata Whereitwascoolerorwarmerin2020comparedwiththemiddleofthe20thcentury –1˚C 0˚ +1˚ +2˚ +3˚ Nodata Whereitwascoolerorwarmerin2020comparedwiththemiddleofthe20thcentury –1˚C 0˚ +1˚ +2˚ +3˚ Nodata Thiswarmingisunprecedentedinrecentgeologichistory.Afamousillustration,firstpublishedin1998andoftencalledthehockey-stickgraph,showshowtemperaturesremainedfairlyflatforcenturies(theshaftofthestick)beforeturningsharplyupward(theblade).It’sbasedondatafromtreerings,icecoresandothernaturalindicators.Andthebasicpicture,whichhaswithstooddecadesofscrutinyfromclimatescientistsandcontrariansalike,showsthatEarthishottertodaythanit’sbeeninatleast1,000years,andprobablymuchlonger.Infact,surfacetemperaturesactuallymaskthetruescaleofclimatechange,becausetheoceanhasabsorbed90percentoftheheattrappedbygreenhousegases.Measurementscollectedoverthelastsixdecadesbyoceanographicexpeditionsandnetworksoffloatinginstrumentsshowthateverylayeroftheoceaniswarmingup.Accordingtoonestudy,theoceanhasabsorbedasmuchheatbetween1997and2015asitdidintheprevious130years.Wealsoknowthatclimatechangeishappeningbecauseweseetheeffectseverywhere.Icesheetsandglaciersareshrinkingwhilesealevelsarerising.Arcticseaiceisdisappearing.Inthespring,snowmeltssoonerandplantsflowerearlier.Animalsaremovingtohigherelevationsandlatitudestofindcoolerconditions.Anddroughts,floodsandwildfireshaveallgottenmoreextreme.Modelspredictedmanyofthesechanges,butobservationsshowtheyarenowcomingtopass.Backtotop.ImageHowmuchagreementisthereamongscientistsaboutclimatechange?There’snodenyingthatscientistsloveagood,old-fashionedargument.Butwhenitcomestoclimatechange,thereisvirtuallynodebate:Numerousstudieshavefoundthatmorethan90percentofscientistswhostudyEarth’sclimateagreethattheplanetiswarmingandthathumansaretheprimarycause.Mostmajorscientificbodies,fromNASAtotheWorldMeteorologicalOrganization,endorsethisview.That’sanastoundinglevelofconsensusgiventhecontrarian,competitivenatureofthescientificenterprise,wherequestionslikewhatkilledthedinosaursremainbitterlycontested.Scientificagreementaboutclimatechangestartedtoemergeinthelate1980s,whentheinfluenceofhuman-causedwarmingbegantoriseabovenaturalclimatevariability.By1991,two-thirdsofearthandatmosphericscientistssurveyedforanearlyconsensusstudysaidthattheyacceptedtheideaofanthropogenicglobalwarming.Andby1995,theIntergovernmentalPanelonClimateChange,afamouslyconservativebodythatperiodicallytakesstockofthestateofscientificknowledge,concludedthat“thebalanceofevidencesuggeststhatthereisadiscerniblehumaninfluenceonglobalclimate.”Currently,morethan97percentofpublishingclimatescientistsagreeontheexistenceandcauseofclimatechange(asdoesnearly60percentofthegeneralpopulationoftheUnitedStates).Sowheredidwegettheideathatthere’sstilldebateaboutclimatechange?Alotofitcamefromcoordinatedmessagingcampaignsbycompaniesandpoliticiansthatopposedclimateaction.Manypushedthenarrativethatscientistsstillhadn’tmadeuptheirmindsaboutclimatechange,eventhoughthatwasmisleading.FrankLuntz,aRepublicanconsultant,explainedtherationaleinaninfamous2002memotoconservativelawmakers:“Shouldthepubliccometobelievethatthescientificissuesaresettled,theirviewsaboutglobalwarmingwillchangeaccordingly,”hewrote.Questioningconsensusremainsacommontalkingpointtoday,andthe97percentfigurehasbecomesomethingofalightningrod.Tobolsterthefalsehoodoflingeringscientificdoubt,somepeoplehavepointedtothingsliketheGlobalWarmingPetitionProject,whichurgedtheUnitedStatesgovernmenttorejecttheKyotoProtocolof1997,anearlyinternationalclimateagreement.Thepetitionproclaimedthatclimatechangewasn’thappening,andevenifitwere,itwouldn’tbebadforhumanity.Since1998,morethan30,000peoplewithsciencedegreeshavesignedit.However,nearly90percentofthemstudiedsomethingotherthanEarth,atmosphericorenvironmentalscience,andthesignatoriesincludedjust39climatologists.Mostwereengineers,doctors,andotherswhosetraininghadlittletodowiththephysicsoftheclimatesystem.Afewwell-knownresearchersremainopposedtothescientificconsensus.Some,likeWillieSoon,aresearcheraffiliatedwiththeHarvard-SmithsonianCenterforAstrophysics,havetiestothefossilfuelindustry.Othersdonot,buttheirassertionshavenotheldupundertheweightofevidence.Atleastoneprominentskeptic,thephysicistRichardMuller,changedhismindafterreassessinghistoricaltemperaturedataaspartoftheBerkeleyEarthproject.Histeam’sfindingsessentiallyconfirmedtheresultshehadsetouttoinvestigate,andhecameawayfirmlyconvincedthathumanactivitieswerewarmingtheplanet.“Callmeaconvertedskeptic,”hewroteinanOp-EdfortheTimesin2012.Mr.Luntz,theRepublicanpollster,hasalsoreversedhispositiononclimatechangeandnowadvisespoliticiansonhowtomotivateclimateaction.Afinalnoteonuncertainty:Denialistsoftenuseitasevidencethatclimatescienceisn’tsettled.However,inscience,uncertaintydoesn’timplyalackofknowledge.Rather,it’sameasureofhowwellsomethingisknown.Inthecaseofclimatechange,scientistshavefoundarangeofpossiblefuturechangesintemperature,precipitationandotherimportantvariables—whichwilldependlargelyonhowquicklywereduceemissions.Butuncertaintydoesnotunderminetheirconfidencethatclimatechangeisrealandthatpeoplearecausingit.Backtotop.ImageDowereallyonlyhave150yearsofclimatedata?Howisthatenoughtotellusaboutcenturiesofchange?Earth’sclimateisinherentlyvariable.Someyearsarehotandothersarecold,somedecadesbringmorehurricanesthanothers,someancientdroughtsspannedthebetterpartofcenturies.Glacialcyclesoperateovermanymillenniums.Sohowcanscientistslookatdatacollectedoverarelativelyshortperiodoftimeandconcludethathumansarewarmingtheplanet?Theansweristhattheinstrumentaltemperaturedatathatwehavetellsusalot,butit’snotallwehavetogoon.Historicalrecordsstretchbacktothe1880s(andoftenbefore),whenpeoplebegantoregularlymeasuretemperaturesatweatherstationsandonshipsastheytraversedtheworld’soceans.Thesedatashowaclearwarmingtrendduringthe20thcentury. +1.2°C Globalaveragetemperaturecomparedwiththemiddleofthe20thcentury +0.75°C +0.50° +0.25° –0.25° 1880 1900 1920 1940 1960 1980 2000 2020 +1.2°C Globalaveragetemperaturecomparedwiththemiddleofthe20thcentury +0.75°C +0.50° +0.25° –0.25° 1880 1920 1960 2000 +1.2°C Globalaveragetemperaturecomparedwiththemiddleofthe20thcentury +0.75°C +0.50° +0.25° –0.25° 1880 1920 1960 2000 ByTheNewYorkTimes Somehavequestionedwhethertheserecordscouldbeskewed,forinstance,bythefactthatadisproportionatenumberofweatherstationsarenearcities,whichtendtobehotterthansurroundingareasasaresultoftheso-calledurbanheatislandeffect.However,researchersregularlycorrectforthesepotentialbiaseswhenreconstructingglobaltemperatures.Inaddition,warmingiscorroboratedbyindependentdatalikesatelliteobservations,whichcoverthewholeplanet,andotherwaysofmeasuringtemperaturechanges.Muchhasalsobeenmadeofthesmalldipsandpausesthatpunctuatetherisingtemperaturetrendofthelast150years.Butthesearejusttheresultofnaturalclimatevariabilityorotherhumanactivitiesthattemporarilycounteractgreenhousewarming.Forinstance,inthemid-1900s,internalclimatedynamicsandlight-blockingpollutionfromcoal-firedpowerplantshaltedglobalwarmingforafewdecades.(Eventually,risinggreenhousegasesandpollution-controllawscausedtheplanettostartheatingupagain.)Likewise,theso-calledwarminghiatusofthe2000swaspartlyaresultofnaturalclimatevariabilitythatallowedmoreheattoentertheoceanratherthanwarmtheatmosphere.Theyearssincehavebeenthehottestonrecord.Still,couldtheentire20thcenturyjustbeonebignaturalclimatewiggle?Toaddressthatquestion,wecanlookatotherkindsofdatathatgivealongerperspective.Researchershaveusedgeologicrecordsliketreerings,icecores,coralsandsedimentsthatpreserveinformationaboutprehistoricclimatestoextendtheclimaterecord.Theresultingpictureofglobaltemperaturechangeisbasicallyflatforcenturies,thenturnssharplyupwardoverthelast150years.Ithasbeenatargetofclimatedenialistsfordecades.However,studyafterstudyhasconfirmedtheresults,whichshowthattheplanethasn’tbeenthishotinatleast1,000years,andprobablylonger.Backtotop.ImageHowdoweknowclimatechangeiscausedbyhumans?Scientistshavestudiedpastclimatechangestounderstandthefactorsthatcancausetheplanettowarmorcool.Thebigonesarechangesinsolarenergy,oceancirculation,volcanicactivityandtheamountofgreenhousegasesintheatmosphere.Andtheyhaveeachplayedaroleattimes.Forexample,300yearsago,acombinationofreducedsolaroutputandincreasedvolcanicactivitycooledpartsoftheplanetenoughthatLondonersregularlyiceskatedontheThames.About12,000yearsago,majorchangesinAtlanticcirculationplungedtheNorthernHemisphereintoafrigidstate.And56millionyearsago,agiantburstofgreenhousegases,fromvolcanicactivityorvastdepositsofmethane(orboth),abruptlywarmedtheplanetbyatleast9degreesFahrenheit,scramblingtheclimate,chokingtheoceansandtriggeringmassextinctions.Intryingtodeterminethecauseofcurrentclimatechanges,scientistshavelookedatallofthesefactors.Thefirstthreehavevariedabitoverthelastfewcenturiesandtheyhavequitelikelyhadmodesteffectsonclimate,particularlybefore1950.Buttheycannotaccountfortheplanet’srapidlyrisingtemperature,especiallyinthesecondhalfofthe20thcentury,whensolaroutputactuallydeclinedandvolcaniceruptionsexertedacoolingeffect.Thatwarmingisbestexplainedbyrisinggreenhousegasconcentrations.Greenhousegaseshaveapowerfuleffectonclimate(seethenextquestionforwhy).AndsincetheIndustrialRevolution,humanshavebeenaddingmoreofthemtotheatmosphere,primarilybyextractingandburningfossilfuelslikecoal,oilandgas,whichreleasescarbondioxide.Bubblesofancientairtrappediniceshowthat,beforeabout1750,theconcentrationofcarbondioxideintheatmospherewasroughly280partspermillion.Itbegantoriseslowlyandcrossedthe300p.p.m.thresholdaround1900.CO2levelsthenacceleratedascarsandelectricitybecamebigpartsofmodernlife,recentlytopping420p.p.m.Theconcentrationofmethane,thesecondmostimportantgreenhousegas,hasmorethandoubled.We’renowemittingcarbonmuchfasterthanitwasreleased56millionyearsago. 30billionmetrictons Carbondioxideemittedworldwide1850-2017 25 India Restofworld 20 China 15 Russia Otherdeveloped 10 EuropeanUnion 5 Developedeconomies Othercountries UnitedStates 1850 1900 1950 2000 30billionmetrictons Carbondioxideemittedworldwide1850-2017 25 India Restofworld 20 China 15 Russia Otherdeveloped 10 EuropeanUnion Developedeconomies 5 Othercountries UnitedStates 1850 1900 1950 2000 30billionmetrictons Carbondioxideemittedworldwide1850-2017 India 20 Restofworld China Russia 10 Otherdeveloped E.U.andU.K. Developedeconomies Othercountries UnitedStates 2017 1850 30billionmetrictons Carbondioxideemittedworldwide1850-2017 Restofworld 20 China Russia 10 E.U.andU.K. Developedeconomies Othercountries UnitedStates 2017 1850 TheNewYorkTimes Theserapidincreasesingreenhousegaseshavecausedtheclimatetowarmabruptly.Infact,climatemodelssuggestthatgreenhousewarmingcanexplainvirtuallyallofthetemperaturechangesince1950.AccordingtothemostrecentreportbytheIntergovernmentalPanelonClimateChange,whichassessespublishedscientificliterature,naturaldriversandinternalclimatevariabilitycanonlyexplainasmallfractionoflate-20thcenturywarming.Anotherstudyputitthisway:Theoddsofcurrentwarmingoccurringwithoutanthropogenicgreenhousegasemissionsarelessthan1in100,000.Butgreenhousegasesaren’ttheonlyclimate-alteringcompoundspeopleputintotheair.Burningfossilfuelsalsoproducesparticulatepollutionthatreflectssunlightandcoolstheplanet.Scientistsestimatethatthispollutionhasmaskeduptohalfofthegreenhousewarmingwewouldhaveotherwiseexperienced.Backtotop.ImageSincegreenhousegasesoccurnaturally,howdoweknowthey’recausingEarth’stemperaturetorise?Greenhousegaseslikewatervaporandcarbondioxideserveanimportantroleintheclimate.Withoutthem,Earthwouldbefartoocoldtomaintainliquidwaterandhumanswouldnotexist!Here’showitworks:theplanet’stemperatureisbasicallyafunctionoftheenergytheEarthabsorbsfromthesun(whichheatsitup)andtheenergyEarthemitstospaceasinfraredradiation(whichcoolsitdown).Becauseoftheirmolecularstructure,greenhousegasestemporarilyabsorbsomeofthatoutgoinginfraredradiationandthenre-emititinalldirections,sendingsomeofthatenergybacktowardthesurfaceandheatingtheplanet.Scientistshaveunderstoodthisprocesssincethe1850s.Greenhousegasconcentrationshavevariednaturallyinthepast.Overmillionsofyears,atmosphericCO2levelshavechangeddependingonhowmuchofthegasvolcanoesbelchedintotheairandhowmuchgotremovedthroughgeologicprocesses.Ontimescalesofhundredstothousandsofyears,concentrationshavechangedascarbonhascycledbetweentheocean,soilandair.Today,however,wearetheonescausingCO2levelstoincreaseatanunprecedentedpacebytakingancientcarbonfromgeologicdepositsoffossilfuelsandputtingitintotheatmospherewhenweburnthem.Since1750,carbondioxideconcentrationshaveincreasedbyalmost50percent.Methaneandnitrousoxide,otherimportantanthropogenicgreenhousegasesthatarereleasedmainlybyagriculturalactivities,havealsospikedoverthelast250years.Weknowbasedonthephysicsdescribedabovethatthisshouldcausetheclimatetowarm.Wealsoseecertaintelltale“fingerprints”ofgreenhousewarming.Forexample,nightsarewarmingevenfasterthandaysbecausegreenhousegasesdon’tgoawaywhenthesunsets.Andupperlayersoftheatmospherehaveactuallycooled,becausemoreenergyisbeingtrappedbygreenhousegasesintheloweratmosphere.Wealsoknowthatwearethecauseofrisinggreenhousegasconcentrations—andnotjustbecausewecanmeasuretheCO2comingoutoftailpipesandsmokestacks.WecanseeitinthechemicalsignatureofthecarboninCO2.Carboncomesinthreedifferentmasses:12,13and14.Thingsmadeoforganicmatter(includingfossilfuels)tendtohaverelativelylesscarbon-13.VolcanoestendtoproduceCO2withrelativelymorecarbon-13.Andoverthelastcentury,thecarboninatmosphericCO2hasgottenlighter,pointingtoanorganicsource.Wecantellit’soldorganicmatterbylookingforcarbon-14,whichisradioactiveanddecaysovertime.Fossilfuelsaretooancienttohaveanycarbon-14leftinthem,soiftheywerebehindrisingCO2levels,youwouldexpecttheamountofcarbon-14intheatmospheretodrop,whichisexactlywhatthedatashow.It’simportanttonotethatwatervaporisthemostabundantgreenhousegasintheatmosphere.However,itdoesnotcausewarming;insteaditrespondstoit.That’sbecausewarmerairholdsmoremoisture,whichcreatesasnowballeffectinwhichhuman-causedwarmingallowstheatmospheretoholdmorewatervaporandfurtheramplifiesclimatechange.Thisso-calledfeedbackcyclehasdoubledthewarmingcausedbyanthropogenicgreenhousegasemissions.Backtotop.ImageWhyshouldwebeworriedthattheplanethaswarmed2°Fsincethe1800s?Acommonsourceofconfusionwhenitcomestoclimatechangeisthedifferencebetweenweatherandclimate.Weatheristheconstantlychangingsetofmeteorologicalconditionsthatweexperiencewhenwestepoutside,whereasclimateisthelong-termaverageofthoseconditions,usuallycalculatedovera30-yearperiod.Or,assomesay:Weatherisyourmoodandclimateisyourpersonality.Sowhile2degreesFahrenheitdoesn’trepresentabigchangeintheweather,it’sahugechangeinclimate.Aswe’vealreadyseen,it’senoughtomelticeandraisesealevels,toshiftrainfallpatternsaroundtheworldandtoreorganizeecosystems,sendinganimalsscurryingtowardcoolerhabitatsandkillingtreesbythemillions.It’salsoimportanttorememberthattwodegreesrepresentstheglobalaverage,andmanypartsoftheworldhavealreadywarmedbymorethanthat.Forexample,landareashavewarmedabouttwiceasmuchastheseasurface.AndtheArctichaswarmedbyabout5degrees.That’sbecausethelossofsnowandiceathighlatitudesallowsthegroundtoabsorbmoreenergy,causingadditionalheatingontopofgreenhousewarming.Relativelysmalllong-termchangesinclimateaveragesalsoshiftextremesinsignificantways.Forinstance,heatwaveshavealwayshappened,buttheyhaveshatteredrecordsinrecentyears.InJuneof2020,atowninSiberiaregisteredtemperaturesof100degrees.AndinAustralia,meteorologistshaveaddedanewcolortotheirweathermapstoshowareaswheretemperaturesexceed125degrees.Risingsealevelshavealsoincreasedtheriskoffloodingbecauseofstormsurgesandhightides.Thesearetheforeshocksofclimatechange.Andweareinformorechangesinthefuture—upto9degreesFahrenheitofaverageglobalwarmingbytheendofthecentury,intheworst-casescenario.Forreference,thedifferenceinglobalaveragetemperaturesbetweennowandthepeakofthelasticeage,whenicesheetscoveredlargepartsofNorthAmericaandEurope,isabout11degreesFahrenheit.UndertheParisClimateAgreement,whichPresidentBidenrecentlyrejoined,countrieshaveagreedtotrytolimittotalwarmingtobetween1.5and2degreesCelsius,or2.7and3.6degreesFahrenheit,sincepreindustrialtimes.Andeventhisnarrowrangehashugeimplications.Accordingtoscientificstudies,thedifferencebetween2.7and3.6degreesFahrenheitwillverylikelymeanthedifferencebetweencoralreefshangingonorgoingextinct,andbetweensummerseaicepersistingintheArcticordisappearingcompletely.Itwillalsodeterminehowmanymillionsofpeoplesufferfromwaterscarcityandcropfailures,andhowmanyaredrivenfromtheirhomesbyrisingseas.Inotherwords,onedegreeFahrenheitmakesaworldofdifference.Backtotop.ImageIsclimatechangeapartoftheplanet’snaturalwarmingandcoolingcycles?Earth’sclimatehasalwayschanged.Hundredsofmillionsofyearsago,theentireplanetfroze.Fiftymillionyearsago,alligatorslivedinwhatwenowcalltheArctic.Andforthelast2.6millionyears,theplanethascycledbetweeniceageswhentheplanetwasupto11degreescoolerandicesheetscoveredmuchofNorthAmericaandEurope,andmilderinterglacialperiodsliketheonewe’reinnow.Climatedenialistsoftenpointtothesenaturalclimatechangesasawaytocastdoubtontheideathathumansarecausingclimatetochangetoday.However,thatargumentrestsonalogicalfallacy.It’slike“seeingamurderedbodyandconcludingthatpeoplehavediedofnaturalcausesinthepast,sothemurdervictimmustalsohavediedofnaturalcauses,”ateamofsocialscientistswroteinTheDebunkingHandbook,whichexplainsthemisinformationstrategiesbehindmanyclimatemyths.Indeed,weknowthatdifferentmechanismscausedtheclimatetochangeinthepast.Glacialcycles,forexample,weretriggeredbyperiodicvariationsinEarth’sorbit,whichtakeplaceovertensofthousandsofyearsandchangehowsolarenergygetsdistributedaroundtheglobeandacrosstheseasons.Theseorbitalvariationsdon’taffecttheplanet’stemperaturemuchontheirown.Buttheysetoffacascadeofotherchangesintheclimatesystem;forinstance,growingormeltingvastNorthernHemisphereicesheetsandalteringoceancirculation.Thesechanges,inturn,affectclimatebyalteringtheamountofsnowandice,whichreflectsunlight,andbychanginggreenhousegasconcentrations.ThisisactuallypartofhowweknowthatgreenhousegaseshavetheabilitytosignificantlyaffectEarth’stemperature.Foratleastthelast800,000years,atmosphericCO2concentrationsoscillatedbetweenabout180partspermillionduringiceagesandabout280p.p.m.duringwarmerperiods,ascarbonmovedbetweenoceans,forests,soilsandtheatmosphere.Thesechangesoccurredinlockstepwithglobaltemperatures,andareamajorreasontheentireplanetwarmedandcooledduringglacialcycles,notjustthefrozenpoles.Today,however,CO2levelshavesoaredto420p.p.m.—thehighestthey’vebeeninatleastthreemillionyears.TheconcentrationofCO2isalsoincreasingabout100timesfasterthanitdidattheendofthelasticeage.Thissuggestssomethingelseisgoingon,andweknowwhatitis:SincetheIndustrialRevolution,humanshavebeenburningfossilfuelsandreleasinggreenhousegasesthatareheatingtheplanetnow(seeQuestion5formoredetailsonhowweknowthis,andQuestions4and8forhowweknowthatothernaturalforcesaren’ttoblame).Overthenextcenturyortwo,societiesandecosystemswillexperiencetheconsequencesofthisclimatechange.Butouremissionswillhaveevenmorelastinggeologicimpacts:Accordingtosomestudies,greenhousegaslevelsmayhavealreadywarmedtheplanetenoughtodelaytheonsetofthenextglacialcycleforatleastanadditional50,000years.ImageHowdoweknowglobalwarmingisnotbecauseofthesunorvolcanoes?ThesunistheultimatesourceofenergyinEarth’sclimatesystem,soit’sanaturalcandidateforcausingclimatechange.Andsolaractivityhascertainlychangedovertime.Weknowfromsatellitemeasurementsandotherastronomicalobservationsthatthesun’soutputchangeson11-yearcycles.Geologicrecordsandsunspotnumbers,whichastronomershavetrackedforcenturies,alsoshowlong-termvariationsinthesun’sactivity,includingsomeexceptionallyquietperiodsinthelate1600sandearly1800s.Weknowthat,from1900untilthe1950s,solarirradianceincreased.Andstudiessuggestthatthishadamodesteffectonearly20thcenturyclimate,explainingupto10percentofthewarmingthat’soccurredsincethelate1800s.However,inthesecondhalfofthecentury,whenthemostwarmingoccurred,solaractivityactuallydeclined.Thisdisparityisoneofthemainreasonsweknowthatthesunisnotthedrivingforcebehindclimatechange.Anotherreasonweknowthatsolaractivityhasn’tcausedrecentwarmingisthat,ifithad,allthelayersoftheatmosphereshouldbeheatingup.Instead,datashowthattheupperatmospherehasactuallycooledinrecentdecades—ahallmarkofgreenhousewarming.Sohowaboutvolcanoes?Eruptionscooltheplanetbyinjectingashandaerosolparticlesintotheatmospherethatreflectsunlight.We’veobservedthiseffectintheyearsfollowinglargeeruptions.Therearealsosomenotablehistoricalexamples,likewhenIceland’sLakivolcanoeruptedin1783,causingwidespreadcropfailuresinEuropeandbeyond,andthe“yearwithoutasummer,”whichfollowedthe1815eruptionofMountTamborainIndonesia.Sincevolcanoesmainlyactasclimatecoolers,theycan’treallyexplainrecentwarming.However,scientistssaythattheymayalsohavecontributedslightlytorisingtemperaturesintheearly20thcentury.That’sbecausetherewereseverallargeeruptionsinthelate1800sthatcooledtheplanet,followedbyafewdecadeswithnomajorvolcaniceventswhenwarmingcaughtup.Duringthesecondhalfofthe20thcentury,though,severalbigeruptionsoccurredastheplanetwasheatingupfast.Ifanything,theytemporarilymaskedsomeamountofhuman-causedwarming.Thesecondwayvolcanoescanimpactclimateisbyemittingcarbondioxide.Thisisimportantontimescalesofmillionsofyears—it’swhatkeepstheplanethabitable(seeQuestion5formoreonthegreenhouseeffect).Butbycomparisontomodernanthropogenicemissions,evenbigeruptionslikeKrakatoaandMountSt.Helensarejustadropinthebucket.Afterall,theylastonlyafewhoursordays,whileweburnfossilfuels24-7.Studiessuggestthat,today,volcanoesaccountfor1to2percentoftotalCO2emissions.Backtotop.ImageHowcanwintersandcertainplacesbegettingcolderiftheplanetiswarming?WhenabigsnowstormhitstheUnitedStates,climatedenialistscantrytociteitasproofthatclimatechangeisn’thappening.In2015,SenatorJamesInhofe,anOklahomaRepublican,famouslylobbedasnowballintheSenateashedenouncedclimatescience.Buttheseeventsdon’tactuallydisproveclimatechange.Whiletherehavebeensomememorablestormsinrecentyears,wintersareactuallywarmingacrosstheworld.IntheUnitedStates,averagetemperaturesinDecember,JanuaryandFebruaryhaveincreasedbyabout2.5degreesthiscentury.Ontheflipside,recordcolddaysarebecominglesscommonthanrecordwarmdays.IntheUnitedStates,recordhighsnowoutnumberrecordlowstwo-to-one.Andever-smallerareasofthecountryexperienceextremelycoldwintertemperatures.(Thesametrendsarehappeningglobally.)Sowhat’swiththeblizzards?Weatheralwaysvaries,soit’snosurprisethatwestillhaveseverewinterstormsevenasaveragetemperaturesrise.However,somestudiessuggestthatclimatechangemaybetoblame.OnepossibilityisthatrapidArcticwarminghasaffectedatmosphericcirculation,includingthefast-flowing,high-altitudeairthatusuallyswirlsovertheNorthPole(a.k.a.thePolarVortex).Somestudiessuggestthatthesechangesarebringingmorefrigidtemperaturestolowerlatitudesandcausingweathersystemstostall,allowingstormstoproducemoresnowfall.Thismayexplainwhatwe’veexperiencedintheU.S.overthepastfewdecades,aswellasawintertimecoolingtrendinSiberia,althoughexactlyhowtheArcticaffectsglobalweatherremainsatopicofongoingscientificdebate.ClimatechangemayalsoexplaintheapparentparadoxbehindsomeoftheotherplacesonEarththathaven’twarmedmuch.Forinstance,asplotchofwaterintheNorthAtlantichascooledinrecentyears,andscientistssaytheysuspectthatmaybebecauseoceancirculationisslowingasaresultoffreshwaterstreamingoffameltingGreenland.Ifthiscirculationgrindsalmosttoahalt,asit’sdoneinthegeologicpast,itwouldalterweatherpatternsaroundtheworld.Notallcoldweatherstemsfromsomecounterintuitiveconsequenceofclimatechange.Butit’sagoodreminderthatEarth’sclimatesystemiscomplexandchaotic,sotheeffectsofhuman-causedchangeswillplayoutdifferentlyindifferentplaces.That’swhy“globalwarming”isabitofanoversimplification.Instead,somescientistshavesuggestedthatthephenomenonofhuman-causedclimatechangewouldmoreaptlybecalled“globalweirding.”Backtotop.ImageWildfiresandbadweatherhavealwayshappened.Howdoweknowthere’saconnectiontoclimatechange?ExtremeweatherandnaturaldisastersarepartoflifeonEarth—justaskthedinosaurs.Butthereisgoodevidencethatclimatechangehasincreasedthefrequencyandseverityofcertainphenomenalikeheatwaves,droughtsandfloods.Recentresearchhasalsoallowedscientiststoidentifytheinfluenceofclimatechangeonspecificevents.Let’sstartwithheatwaves.Studiesshowthatstretchesofabnormallyhightemperaturesnowhappenaboutfivetimesmoreoftenthantheywouldwithoutclimatechange,andtheylastlonger,too.Climatemodelsprojectthat,bythe2040s,heatwaveswillbeabout12timesmorefrequent.Andthat’sconcerningsinceextremeheatoftencausesincreasedhospitalizationsanddeaths,particularlyamongolderpeopleandthosewithunderlyinghealthconditions.Inthesummerof2003,forexample,aheatwavecausedanestimated70,000excessdeathsacrossEurope.(Human-causedwarmingamplifiedthedeathtoll.)Climatechangehasalsoexacerbateddroughts,primarilybyincreasingevaporation.DroughtsoccurnaturallybecauseofrandomclimatevariabilityandfactorslikewhetherElNiñoorLaNiñaconditionsprevailinthetropicalPacific.ButsomeresearchershavefoundevidencethatgreenhousewarminghasbeenaffectingdroughtssinceevenbeforetheDustBowl.Anditcontinuestodosotoday.Accordingtooneanalysis,thedroughtthatafflictedtheAmericanSouthwestfrom2000to2018wasalmost50percentmoreseverebecauseofclimatechange.Itwastheworstdroughttheregionhadexperiencedinmorethan1,000years.Risingtemperatureshavealsoincreasedtheintensityofheavyprecipitationeventsandthefloodingthatoftenfollows.Forexample,studieshavefoundthat,becausewarmerairholdsmoremoisture,HurricaneHarvey,whichstruckHoustonin2017,droppedbetween15and40percentmorerainfallthanitwouldhavewithoutclimatechange.It’sstillunclearwhetherclimatechangeischangingtheoverallfrequencyofhurricanes,butitismakingthemstronger.Andwarmingappearstofavorcertainkindsofweatherpatterns,likethe“MidwestWaterHose”eventsthatcauseddevastatingfloodingacrosstheMidwestin2019.It’simportanttorememberthatinmostnaturaldisasters,therearemultiplefactorsatplay.Forinstance,the2019Midwestfloodsoccurredafterarecentcoldsnaphadfrozenthegroundsolid,preventingthesoilfromabsorbingrainwaterandincreasingrunoffintotheMissouriandMississippiRivers.Thesewaterwayshavealsobeenreshapedbyleveesandotherformsofriverengineering,someofwhichfailedinthefloods.Wildfiresareanotherphenomenonwithmultiplecauses.Inmanyplaces,fireriskhasincreasedbecausehumanshaveaggressivelyfoughtnaturalfiresandpreventedIndigenouspeoplesfromcarryingouttraditionalburningpractices.Thishasallowedfueltoaccumulatethatmakescurrentfiresworse.However,climatechangestillplaysamajorrolebyheatinganddryingforests,turningthemintotinderboxes.Studiesshowthatwarmingisthedrivingfactorbehindtherecentincreasesinwildfires;oneanalysisfoundthatclimatechangeisresponsiblefordoublingtheareaburnedacrosstheAmericanWestbetween1984and2015.Andresearcherssaythatwarmingwillonlymakefiresbiggerandmoredangerousinthefuture.Backtotop.ImageHowbadaretheeffectsofclimatechangegoingtobe?Itdependsonhowaggressivelyweacttoaddressclimatechange.Ifwecontinuewithbusinessasusual,bytheendofthecentury,itwillbetoohottogooutsideduringheatwavesintheMiddleEastandSouthAsia.DroughtswillgripCentralAmerica,theMediterraneanandsouthernAfrica.Andmanyislandnationsandlow-lyingareas,fromTexastoBangladesh,willbeovertakenbyrisingseas.Conversely,climatechangecouldbringwelcomewarmingandextendedgrowingseasonstotheupperMidwest,Canada,theNordiccountriesandRussia.Farthernorth,however,thelossofsnow,iceandpermafrostwillupendthetraditionsofIndigenouspeoplesandthreateninfrastructure.It’scomplicated,buttheunderlyingmessageissimple:uncheckedclimatechangewilllikelyexacerbateexistinginequalities.Atanationallevel,poorercountrieswillbehithardest,eventhoughtheyhavehistoricallyemittedonlyafractionofthegreenhousegasesthatcausewarming.That’sbecausemanylessdevelopedcountriestendtobeintropicalregionswhereadditionalwarmingwillmaketheclimateincreasinglyintolerableforhumansandcrops.Thesenationsalsooftenhavegreatervulnerabilities,likelargecoastalpopulationsandpeoplelivinginimprovisedhousingthatiseasilydamagedinstorms.Andtheyhavefewerresourcestoadapt,whichwillrequireexpensivemeasureslikeredesigningcities,engineeringcoastlinesandchanginghowpeoplegrowfood.Already,between1961and2000,climatechangeappearstohaveharmedtheeconomiesofthepoorestcountrieswhileboostingthefortunesofthewealthiestnationsthathavedonethemosttocausetheproblem,makingtheglobalwealthgap25percentbiggerthanitwouldotherwisehavebeen.Similarly,theGlobalClimateRiskIndexfoundthatlowerincomecountries—likeMyanmar,HaitiandNepal—rankhighonthelistofnationsmostaffectedbyextremeweatherbetween1999and2018.Climatechangehasalsocontributedtoincreasedhumanmigration,whichisexpectedtoincreasesignificantly.Evenwithinwealthycountries,thepoorandmarginalizedwillsufferthemost.Peoplewithmoreresourceshavegreaterbuffers,likeair-conditionerstokeeptheirhousescoolduringdangerousheatwaves,andthemeanstopaytheresultingenergybills.Theyalsohaveaneasiertimeevacuatingtheirhomesbeforedisasters,andrecoveringafterward.Lowerincomepeoplehavefeweroftheseadvantages,andtheyarealsomorelikelytoliveinhotterneighborhoodsandworkoutdoors,wheretheyfacethebruntofclimatechange.Theseinequalitieswillplayoutonanindividual,community,andregionallevel.A2017analysisoftheU.S.foundthat,underbusinessasusual,thepoorestone-thirdofcounties,whichareconcentratedintheSouth,willexperiencedamagestotalingasmuchas20percentofgrossdomesticproduct,whileothers,mostlyinthenorthernpartofthecountry,willseemodesteconomicgains.SolomonHsiang,aneconomistatUniversityofCalifornia,Berkeley,andtheleadauthorofthestudy,hassaidthatclimatechange“mayresultinthelargesttransferofwealthfromthepoortotherichinthecountry’shistory.”Eventheclimate“winners”willnotbeimmunefromallclimateimpacts,though.Desirablelocationswillfaceaninfluxofmigrants.Andasthecoronaviruspandemichasdemonstrated,disastersinoneplacequicklyrippleacrossourglobalizedeconomy.Forinstance,scientistsexpectclimatechangetoincreasetheoddsofmultiplecropfailuresoccurringatthesametimeindifferentplaces,throwingtheworldintoafoodcrisis.Ontopofthat,warmerweatherisaidingthespreadofinfectiousdiseasesandthevectorsthattransmitthem,liketicksandmosquitoes.Researchhasalsoidentifiedtroublingcorrelationsbetweenrisingtemperaturesandincreasedinterpersonalviolence,andclimatechangeiswidelyrecognizedasa“threatmultiplier”thatincreasestheoddsoflargerconflictswithinandbetweencountries.Inotherwords,climatechangewillbringmanychangesthatnoamountofmoneycanstop.Whatcouldhelpistakingactiontolimitwarming.Backtotop.ImageWhatwillitcosttodosomethingaboutclimatechange,versusdoingnothing?Oneofthemostcommonargumentsagainsttakingaggressiveactiontocombatclimatechangeisthatdoingsowillkilljobsandcrippletheeconomy.Butthisimpliesthatthere’sanalternativeinwhichwepaynothingforclimatechange.Andunfortunately,thereisn’t.Inreality,nottacklingclimatechangewillcostalot,andcauseenormoushumansufferingandecologicaldamage,whiletransitioningtoagreenereconomywouldbenefitmanypeopleandecosystemsaroundtheworld.Let’sstartwithhowmuchitwillcosttoaddressclimatechange.Tokeepwarmingwellbelow2degreesCelsius,thegoaloftheParisClimateAgreement,societywillhavetoreachnetzerogreenhousegasemissionsbythemiddleofthiscentury.Thatwillrequiresignificantinvestmentsinthingslikerenewableenergy,electriccarsandcharginginfrastructure,nottomentioneffortstoadapttohottertemperatures,risingsea-levelsandotherunavoidableeffectsofcurrentclimatechanges.Andwe’llhavetomakechangesfast.Estimatesofthecostvarywidely.Onerecentstudyfoundthatkeepingwarmingto2degreesCelsiuswouldrequireatotalinvestmentofbetween$4trillionand$60trillion,withamedianestimateof$16trillion,whilekeepingwarmingto1.5degreesCelsiuscouldcostbetween$10trillionand$100trillion,withamedianestimateof$30trillion.(Forreference,theentireworldeconomywasabout$88trillionin2019.)Otherstudieshavefoundthatreachingnetzerowillrequireannualinvestmentsrangingfromlessthan1.5percentofglobalgrossdomesticproducttoasmuchas4percent.That’salot,butwithintherangeofhistoricalenergyinvestmentsincountriesliketheU.S.Now,let’sconsiderthecostsofuncheckedclimatechange,whichwillfallhardestonthemostvulnerable.Theseincludedamagetopropertyandinfrastructurefromsea-levelriseandextremeweather,deathandsicknesslinkedtonaturaldisasters,pollutionandinfectiousdisease,reducedagriculturalyieldsandlostlaborproductivitybecauseofrisingtemperatures,decreasedwateravailabilityandincreasedenergycosts,andspeciesextinctionandhabitatdestruction.Dr.Hsiang,theU.C.Berkeleyeconomist,describesitas“deathbyathousandcuts.”Asaresult,climatedamagesarehardtoquantify.Moody’sAnalyticsestimatesthateven2degreesCelsiusofwarmingwillcosttheworld$69trillionby2100,andeconomistsexpectthetolltokeeprisingwiththetemperature.Inarecentsurvey,economistsestimatedthecostwouldequal5percentofglobalG.D.P.at3degreesCelsiusofwarming(ourtrajectoryundercurrentpolicies)and10percentfor5degreesCelsius.Otherresearchindicatesthat,ifcurrentwarmingtrendscontinue,globalG.D.P.percapitawilldecreasebetween7percentand23percentbytheendofthecentury—aneconomicblowequivalenttomultiplecoronaviruspandemicseveryyear.Andsomefearthesearevastunderestimates.Already,studiessuggestthatclimatechangehasslashedincomesinthepoorestcountriesbyasmuchas30percentandreducedglobalagriculturalproductivityby21percentsince1961.Extremeweathereventshavealsorackedupalargebill.In2020,intheUnitedStatesalone,climate-relateddisasterslikehurricanes,droughts,andwildfirescausednearly$100billionindamagestobusinesses,propertyandinfrastructure,comparedtoanaverageof$18billionperyearinthe1980s.Giventhesteeppriceofinaction,manyeconomistssaythataddressingclimatechangeisabetterdeal.It’slikethatoldsaying:anounceofpreventionisworthapoundofcure.Inthiscase,limitingwarmingwillgreatlyreducefuturedamageandinequalitycausedbyclimatechange.Itwillalsoproduceso-calledco-benefits,likesavingonemillionliveseveryyearbyreducingairpollution,andmillionsmorefromeatinghealthier,climate-friendlydiets.SomestudiesevenfindthatmeetingtheParisAgreementgoalscouldcreatejobsandincreaseglobalG.D.P.And,ofcourse,reininginclimatechangewillsparemanyspeciesandecosystemsuponwhichhumansdepend—andwhichmanypeoplebelievetohavetheirowninnatevalue.Thechallengeisthatweneedtoreduceemissionsnowtoavoiddamageslater,whichrequiresbiginvestmentsoverthenextfewdecades.Andthelongerwedelay,themorewewillpaytomeettheParisgoals.Onerecentanalysisfoundthatreachingnet-zeroby2050wouldcosttheU.S.almosttwiceasmuchifwewaiteduntil2030insteadofactingnow.ButevenifwemisstheParistarget,theeconomicsstillmakeastrongcaseforclimateaction,becauseeveryadditionaldegreeofwarmingwillcostusmore—indollars,andinlives.Backtotop.VeronicaPenneycontributedreporting.IllustrationphotographsbyEstherHorvath,MaxWhittaker,DavidMauriceSmithandTaliaHermanforTheNewYorkTimes;EstherHorvath/Alfred-Wegener-InstitutAdvertisementContinuereadingthemainstory